NASA Evaluates Rescue Options for Hubble Mission

July 31, 2006 by John Copella

Details are starting to emerge about potential crew rescue options for the Hubble Servicing Mission (HSM), currently designated STS-125.NASA Documents, which confirm STS-125 is the HST-SM04 mission, show the flight – currently scheduled to launch with Shuttle Discovery, NET (No Earlier Than) April 11, 2008 – has a rescue mission requirement under evaluation.*Information taken from the on-going STS-125 and HSM LON update pages on L2 – which hold documentation and info relating to both primary and LON missions*

While a crew rescue option using a backup Shuttle has been a characteristic of both post-STS-107 flights so far, the HSM presents unique problems, outlined in a fascinating document.

For a worst-case scenario in which the HSM Orbiter is found to be fatally damaged after reaching orbit, NASA is starting to refine its planning regarding timelines, rendezvous and docking options, crew and suit transfer, required waivers and flight hardware modifications, and procedures for disposal – or unmanned re-entry attempt – of the damaged Orbiter.Following the loss of Columbia on STS-107, NASA developed contingency plans and procedures that called for use of the International Space Station (ISS) as a safe haven. In the event that unrepairable damage to the Orbiter’s heat shield was detected on-orbit, NASA would direct the Orbiter to rendezvous and dock with the ISS, and transfer the crew aboard the Station until a rescue flight (called a Launch-On-Need, or LON, mission) could be launched to retrieve the crew and return them to earth.

However, for the HSM, because the Hubble Space Telescope (HST) orbits at a higher altitude and a different inclination than the ISS, a safe haven option at the Station is not possible.

For the HSM, the Orbiter simply does not carry enough fuel to effect the required orbital changes necessary to conduct an ISS rendezvous. As a result, NASA is looking at ways of keeping the crew of the stranded Orbiter alive, while a second rescue Orbiter is quickly prepped for flight, launched, and maneuvered to the disabled craft.

Complicating matters is the fact that pad 39-B is designated to be transferred to the CEV/CLV project for facility modifications in early 2007, leaving only a single pad (LC-39A) available to support both the HSM launch, and, if needed, a subsequent rescue mission approximately two weeks later.

According to NASA documents obtained by NASASpaceflight.com, NASA would launch a rescue mission only if unrepairable damage to the HSM orbiter’s heat shield is detected on-orbit. Because of the limited amount of reactants for the Orbiter’s fuel cells, which supply power, oxygen, and drinking water for the crew, NASA would need to decide by flight day 4 of the HSM whether or not to mount a rescue mission.

This would require NASA, as it has done on the two post-STS-107 flights, to give initial priority to inspection and evaluation of the Orbiter’s heat shield using the Orbiter Boom Sensor System (OBSS).

While the damaged orbiter would have to revert to ‘lifeboat’ mode, shutting down as many systems as possible, the rescue Orbiter would be readied for launch. Without pad 39-B, NASA would have to bring in a record Launch-To-Launch turnaround, described as the ‘L-t-L’ ratio, knocking days off the re-availability of a pad following a launch by half.

Three options are currently being considered for docking the two Orbiters, and for transferring the crew from the crippled HSM vehicle to the rescue Orbiter. In the first, and apparently preferred, option, the Remote Manipulator System on the HSM Orbiter would grapple the Shuttle Crew Rescue (SCR) Orbiter. The SCR RMS would be used to transfer the crew, suited for EVA, from the HSM Orbiter to the SCR vehicle. This unique dual-RMS operation would eliminate the need for labor-intensive stationkeeping.

Two EVAs would occur on Flight Day 4. During the first EVA, the flight crew member responsible for RMS operation on the HSM flight would be transferred to the SCR. In addition, the SCR RMS would be used to transfer 4 Launch and Entry Suits (LES) from the HSM vehicle to the SCR, and two additional Extravehicular Mobility Units (EMUs) would be transferred from the SCR to the HSM.

The LES and EMU transfers are needed in order to provide enough suits for the crew from the crippled orbiter to make the EVA transfer from one vehicle to another, as well as to provide each crew member with the proper pressurized launch and entry suit for entry in the SCR vehicle. A second EVA would be used to transfer two additional LES from the HSM to the SCR, transfer an additional EMU from the SCR to the HSM, and transfer one EVA team and the pilot from the HSM to the SCR.

On flight day 5, the two vehicles would ungrapple, and the SCR RMS would again be used to transfer the two remaining LES from the HSM to the SCR, and to transfer two additional and unneeded EMUs from the SCR to the HSM, creating precious room for the HSM crew on the SCR middeck. A third and final EVA would be performed to transfer the remaining three crew members – the second EVA team and the HSM commander – from the HSM to the SCR.

At the completion of this complex ballet, the HSM Orbiter would be vacant of its crew and their LES, and would serve as storage for two additional EMUs that would be discarded along with, sadly, the crippled Orbiter itself. The SCR, in contrast, will be cramped – 11 crew members will be aboard, along with 4 bulky EMUs and 11 LES. Flight Day 6 would be used for stowage of loose items in the crew module and preparations for deorbit. Landing would occur on Flight Day 7.

In order to make this scheme work, NASA is considering a number of equipment modifications and waivers to its existing procedures and flight rules. With a maximum of four seats installed on the flight deck, the additional seven crew members from the HSM will have to be squeezed into seats on the middeck of the SCR.

One seat on the middeck would be installed in the upright position. The remaining six would be installed on their backs (recumbent). Three of those six recumbent seats would require new flight hardware for attaching to the aft middeck floor.

This configuration does not provide enough room for the Shuttle ergometer, and therefore NASA is looking at ways to provide the crew with the required exercise needed to offset the adverse physiological effects of several days in microgravity without the device.

Other configuration changes that are being considered by NASA include waiving the requirement for emergency bailout hardware, which would mean launching without the emergency egress pole and its associated equipment, and possible waivers for the 20g deceleration load limits on some flight hardware. NASA is also evaluating what, if any, modifications will need to be made to the Shuttle simulators at JSC in order to accommodate scenarios that involve two crews operating two Orbiters simultaneously.

A look at the overall timeline shows that NASA plans on the HSM being able to survive at least 17 days on-orbit, with additional contingency days possible if strict power and consumable conservation measures are adopted. Given this margin, which NASA believes amounts to about six days, the space agency believes it has enough time to carry out an exhaustive inspection of the HSM Orbiter’s thermal protection system (TPS), and still leave enough time to roll-out and launch a rescue mission.

NASA believes safe separation and disposal of the damaged orbiter is feasible based on engineering it performed to develop the ISS safe haven procedures. Prior to crew egress, the HSM vehicle would be placed in a mode where its attitude would be constant and stable, the HSM Orbiter’s Orbital Maneuvering System (OMS) engines would be armed, and the SCR would ungrapple from the HSM.

After final crew egress and relocation to the SCR, Houston would then send commands to the HSM Orbiter to fire its OMS engines, triggering entry and destruction of the orbiter due to what is presumed to be fatal TPS damage. However, the Orbiter may also be saved via the Autonomous Orbiter Rapid Prototype (AORP), or Remote Controlled Orbiter IFM cable, for a controlled re-entry in an attempt to save the vehicle.

STS-125 isn’t expected to make the working manifest until the end of the year, and a supporting LON mission is very unlikely to be mentioned at that time, given the route NASA wishes to take in the lead up to this mission.

On-going improvements to on orbit repair techniques is what NASA wishes to be the rationale for removing the supporting LON flights from the manifest – and from dismissing its need for one to associated with STS-125. However, should the agency suffer a setback, and still require to service Hubble, a one-of-its-kind rescue mission may be required.